Press Release

Older than the Sun: Astronomers find new clues to the origin of interstellar comet 3I/ATLAS

6 July 2026

VLT image of interstellar comet 3I/ATLAS (18 January 2026) (Credit: ESO/O. Hainaut)

Astronomers have used the European Southern Observatory's Very Large Telescope (ESO's VLT) to study the composition of 3I/ATLAS, the brightest interstellar object ever seen, in detail. By measuring specific chemical fingerprints — the first observations of this kind for a comet that formed outside the Solar System — they found that 3I/ATLAS likely originated in the outskirts of an old star system. The findings shine new light on the formation history of this comet, indicating that it may be much older than the Sun.

Interstellar comets are icy objects formed around a star other than the Sun that occasionally wander into our Solar System. "They are sort of fossils from a planetary formation process that happened very far away, but that we get the chance to study from much closer," says astronomer Cyrielle Opitom, a researcher at the University of Edinburgh, United Kingdom. Together with Jean Manfroid and Damien Hutsemékers of the University of Liège, Belgium, Opitom led a study of 3I/ATLAS published today in Nature Astronomy.

3I/ATLAS is the third interstellar object ever discovered, after 1I/ʻOumuamua and 2I/Borisov. It was found as it was approaching the Sun, spending enough time in our Solar System for astronomers to study it in detail. While it was difficult to measure the composition of the first two interstellar objects — in the first astronomers didn’t detect gas and the second was too faint — this was not the case for 3I/ATLAS. Thanks to the object's unprecedented brightness, Opitom, Manfroid, Hutsemékers and their team were able to measure the comet's isotopic ratios: the relative amounts of different forms of the same element.

Using the UVES instrument on ESO's VLT, the team measured ratios of carbon and nitrogen isotopes in cyanide molecules present in the gas around the comet. These ratios are known to be a good indicator of a comet’s origin, as they are very sensitive to the physical conditions in the formation environment and are not expected to change much as the comet travels on through space.

“Unlike comets from our Solar System, this interstellar visitor carries unusually high carbon and nitrogen isotopic ratios,” explains Aravind Krishnakumar, a researcher at the University of Liège and co-author on the new study. A similar study led by Martin Cordiner at the NASA Goddard Space Flight Center, US, that was published late last month in Nature, found a similar isotopic ratio of carbon, as well as elevated levels of deuterium, also called heavy hydrogen [1]. The study used data from the James Webb Space Telescope, a joint project of the US, European and Canadian space agencies.

Overall, the findings by Opitom’s team indicate that the comet likely formed in the outer regions around an old, ‘low-metallicity’ star. A low-metallicity star is one with few elements heavier than helium in its composition, that is thought to have formed when the Universe was much younger — and less chemically rich — than it is now. The team suspects that 3I/ATLAS therefore originated around a star much older than the Sun. “3I/ATLAS is a really exciting opportunity to probe the composition of another planetary system, one that formed long before our Sun and Solar System even existed," says co-author Rosemary Dorsey, a researcher at the University of Helsinki, Finland. Evidence from the studies by the different teams points to 3I/ATLAS being more than twice as old as the Sun.

As 3I/ATLAS moves away from the Sun and gets progressively fainter, its observations at the VLT are also nearing their end. ESO's upcoming Extremely Large Telescope (ELT) will allow similar measurements for future interstellar objects, including those less bright than 3I/ATLAS. "The field of interstellar objects is still very new, and we do not really know what to expect. Every time a new one is discovered, we have new surprises," Opitom concludes.

Notes

[1] A team lead by Salazar-Manzano and Paneque-Carreño used the Atacama Large Millimeter/submillimeter Array (ALMA), in which ESO is a partner, to measure deuterated (or semi-heavy) water in 3I/ATLAS. They also found elevated levels of this type of water compared to those found in Solar System comets.

More information

This research was presented in a paper to appear in Nature Astronomy (doi:10.1038/s41550-026-02921-7).

The team is composed of C. Opitom (Institute for Astronomy, University of Edinburgh, Royal Observatory, UK [Edinburgh]), J. Manfroid (STAR Institute, University of Liège, Belgium [STAR]), D. Hutsemékers (STAR), E. Jehin (STAR), M. M. Knight (Volgenau Department of Physics, United States Naval Academy, Annapolis, MD, USA), K. Aravind (STAR), L. Ferellec (Faculty of Science and Engineering, Northumbria University, Newcastle, UK), D. Bodewits (Physics Department, Edmund C. Leach Science Center, Auburn University, AL, USA), V. V. Guzmán (Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Santiago, Chile), M. Cordiner (Department of Physics, Catholic University of America, Washington, DC, USA and Astrochemistry Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA), R. C. Dorsey (Department of Physics, University of Helsinki, Finland), F. La Forgia (Department of Physics and Astronomy, University of Padova, Italy), M. Lippi (INAF - Osservatorio Astrofisico di Arcetri, Firenze, Italy), B. P. Murphy (Edinburgh), C. Snodgrass (Edinburgh).

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Links

• Research paper
• Chasing Starlight video of 3I/ATLAS, featuring Cyrielle Opitom (from December 2025)
• Photos of the VLT
• Find out more about ESO's Extremely Large Telescope on our dedicated website and press kit
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Contacts

Cyrielle Opitom
School of Physics and Astronomy, University of Edinburgh
Edinburgh, United Kingdom
Tel: +44 (0)131 668 8350
Email: copi@roe.ac.uk

Aravind Krishnakumar
Space sciences, Technologies & Astrophysics Research (STAR) Institute, University of Liège
Liège, Belgium
Email: aravind139@gmail.com

Rosemary Dorsey
University of Helsinki
Helsinki, Finland
Email: rosemary.dorsey@helsinki.fi

Bárbara Ferreira
ESO Media Manager
Garching bei München, Germany
Tel: +49 89 3200 6670
Cell: +49 151 241 664 00
Email: press@eso.org

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